1. Field of the Invention
The present invention relates to a spray-type dampening water supply apparatus for an offset printing press, and more particularly to a spray-type dampening water supply apparatus having nozzles that can be prevented from being contaminated with dirt at a portion surrounding and adjacent to the orifice of each nozzle.
2. Description of the Background
In offset printing, printing is carried out using a printing plate having a uniform surface formed of image regions that are lipophilic and non-image regions that are hydrophilic. First, dampening water and oil-based ink are supplied to the plate surface so that the ink adheres to only the image regions by the interactive repulsion between the water and oil. Then, this inked image is printed on paper via a blanket.
In offset printing presses, it is known to use a spray-type dampening water supply apparatus that supplies dampening water to the peripheral surface of a roller extending to the printing plate, from nozzles. The orifice of each nozzle has a very small oval hole in order to provide very fine water drops and adjust the spraying with precision. A filter is located in a pipe through which the dampening water is fed from a reservoir to the nozzles, to block small-size foreign matter. This conventional technology is exemplified by NEWSPAPER PRINTING MANUAL published Apr. 10, 1997 by the Japan Newspaper Association, pages 75-76 (hereinafter called Prior Art 1) and Japanese Utility Model Registration Gazette No. 2602799 (hereinafter called Prior Art 2).
In many spray-type dampening water supply apparatuses of the type according to Prior Art 1, spray nozzles Q10 of a shape shown in FIG. 13 of the accompanying drawings are used. Each nozzle Q10 has a single, generally C-shaped groove Q17 engraved in the end surface Q16, and a nozzle tip Q12 having an orifice Q13 projecting centrally from the groove bottom Q18 toward the nozzle end surface Q16, and terminating short of this nozzle end surface Q16. Prior Art 1 shows swirl QH.
In Prior Art 2, as shown in FIG. 14, a pipe, which dampening water is made to flow through from a dampening water reservoir B1 toward a spray unit Q201 under pressure, has a first valve V1 located upstream of the spray unit Q201 for opening and closing the pipe and a second valve V2 for opening the pipe to the outside in order to suck in air. In a pipe leading to the reservoir B1 to be connected downstream of the nozzle Q20, a pipe leading to a decompression container B2, and a pipe leading to a compression container B3, there are respectively located a sixth valve V1a for opening and closing the pipe with respect to the reservoir B1, the second valve V2 for opening and closing the pipe with respect to the decompression container B2 decompressed to a pressure below atmospheric pressure by a decompression pump U2, and a third valve V3 containing cleaning liquid M and compressed to a pressure above atmospheric pressure by a compression pump U3. Note that in Prior Art 2, a filter QF is present.
In Prior Art 2, to remove foreign matter jammed in the orifice Q23, an electromagnetic valve V5 in the spray unit Q201 is closed to stop spraying of dampening water and, in the meantime, the first valve V1 and the sixth valve V1a disposed upstream and downstream, respectively, of the spray unit Q201 are closed. Then, when the second valve V2 is opened to communicate the associated pipe with the decompression container B2, the pressure in the same pipe is decreased, and when the electromagnetic valve V5 is opened the associated pipe is communicated to outside (atmosphere) to suck in air from the orifice Q23. As a result, any foreign matter jammed in the orifice Q23, together with the residual dampening water in the vicinity of the orifice Q23 and in the pipe, is urged to be introduced into the pipe. Subsequently, when the fourth valve V4 is opened, the residual liquid containing foreign matter in the pipe is sucked into the decompression container B2. This procedure is repeated several times in an effort to remove the foreign matter jammed in the orifice Q23.
If this removing of the foreign matter jammed in the orifice Q23 is unsuccessful, the first valve V1 and the sixth valve V1a disposed upstream and downstream, respectively, of the spray unit Q201 are closed and, at the same time, a breaker plate Q271 is moved toward and short of the orifice Q23. Then, when the third valve V3 is opened to communicate the associated pipe to the compression container B3, the cleaning liquid M to be supplied to the nozzle Q20 from the compression container B3 is sprayed so that the foreign matter jammed in the orifice Q23 is released from the orifice Q23. At the same time, the cleaning liquid M sprayed to the breaker plate Q271 is dispersed to clean away the dirt adhered to the circumference of the nozzle Q20.
However, Prior Art 1 and Prior Art 2 encountered the following problems. With the nozzle according to Prior Art 1 disclosed in the NEWSPAPER PRINTING MANUAL, dampening water is sprayed from the generally C-shaped groove engraved in the nozzle end surface. Generally, in circulating spouted fluid, which is different in velocity from the surrounding fluid, pressure is lowered so as to draw in the surrounding fluid due to the velocity difference. Accordingly, around spouted fluid, on every occasion dampening water is sprayed, an atmosphere contaminated with ink mist, paper powder, etc. strikes the nozzle tip and the neighboring portion as a complicated eddy flow, thereby instantly making their surfaces dirty and causing standing water as residual dampening water around the nozzle tip. After termination of printing, the spraying status would be deteriorated due to the dirt deposited as the standing residual water vapor. Consequently, to obtain an optimized amount of spray of dampening water, these prior art technologies require meticulous adjustments in spray amount of dampening water and also periodical maintenance and cleaning, which are laborious and time-consuming.
According to Prior art 2 disclosed in Japanese Utility Model Registration Gazette No. 2602799, in an adjusted dampening water supply spray-type apparatus, a filter in the pipe blocks foreign matter in an attempt to prevent the orifice from becoming clogged with foreign matter. Instead, dampening water mist, ink mist and paper powder float in the damping-water spraying space. The residual dampening water containing these substances is mixed into the standing liquid adhered around the nozzle and vapor to cause deposited foreign matter after termination of printing, so that the orifice tends to become clogged with dirt as the deposited foreign matter.
Further, during cleaning of the nozzle, the above-mentioned foreign matter would enter the nozzle from the orifice to clog the orifice or enter the oval hole of the nozzle end surface to be caught inside the orifice at its small-diameter side during subsequent spraying. To cope with this clogging, reduction of the pressure in the pipe could be considered to suck the foreign matter from the orifice. However, because of the smallness of the orifice, only an inadequate sucking force can be expected.
In addition, the foreign matter adhered around the nozzle tends to be attracted into the orifice to increase clogging of the orifice, but the foregoing prior art technologies could not solve this problem.
As described above, a cleaning liquid is sprayed toward the breaker plate, which is disposed immediately upstream of the orifice, from the compression container in an attempt to wash away foreign matter that has entered and clogged the orifice and also wash around the circumference of the nozzle with the cleaning liquid reflected on the breaker plate. However, partly because the pressure of the compressed cleaning liquid is lowered due to the smallness of the orifice diameter, the washing power would be attenuated to such a level that the foreign matter could only be incompletely removed.
Further, much of the sprayed cleaning liquid is scattered along the surface of the breaker plate rather than reflecting on the breaker plate, so that an adequate degree of spraying power with respect to the nozzle confronting the breaker plate could not be guaranteed. Furthermore, because after termination of this cleaning, residual water occurs around the nozzle and then vaporizes to cause deposited dirt, an intended cleaning effect cannot be expected.
For removing foreign matter jammed in and dirt adhered around the orifice, it requires a decompression container, a compression container, a breaker plate and a control unit for controlling these parts, so that the whole apparatus is large in size and expensive.